Portable, handheld microanalytical systems, which have been termed “chemical laboratories on a chip,” are being developed to enable the rapid and sensitive detection of particular chemicals, including pollutants, high explosives, and chemical and biological warfare agents. These microanalytical systems should provide a high chemical selectivity, to discriminate against potential background interferents, and the ability to perform the chemical analysis on a short time scale with high sensitivity. In addition, low electrical power consumption is needed for prolonged field use. See G. C. Frye-Mason et al., Proc. Micro Total Analysis Systems 2000, Kluwer Academic Publisher, Dordrecht, The Netherlands, pp. 229–232, (2000) and G. C. Frye-Mason et al., Proc. Micro Total Analysis Systems 2001, Kluwer Academic Publisher, Dordrecht, The Netherlands, pp. 658–660, (2001).
In almost any chemical analysis system, there are three important stages: sample collection, sample separation, and sample identification. Current gas-phase microanalytical systems typically comprise a gas chromatography column to separate the chemical species, or analytes, in a gas sample and a detector to identify the separated species. Such microanalytical systems can also include a chemical preconcentrator for sample collection. The chemical preconcentrator serves the important function of collecting and concentrating the chemical analytes on a sorptive material at the inlet of the microanalytical system. The chemical preconcentrator can deliver an extremely sharp sample plug to the downstream gas chromatograph by taking advantage of the rapid, efficient heating of the sorbed analytes with a low-heat capacity, low-loss microhotplate. The very narrow temporal plug improves separations, and therefore the signal-to-noise ratio and sensitivity to the particular chemical species of interest. In particular, selective analyte preconcentration is an essential step for early-warning, trace chemical detection in real-world, high-consequence environments where a high background of potentially interfering compounds exists.
Previous microfabricated chemical preconcentrators have typically used a heated planar membrane suspended from a substrate as the microhotplate, wherein the sorptive material is disposed as a layer on a surface of the membrane to sorb the chemical species from a gas stream. The sorptive material thereby collects and concentrates the sample, and then the heated membrane thermally desorbs the sample in a short pulse for subsequent separation. See U.S. Pat. No. 6,171,378 to Manginell et al., which is incorporated herein by reference. Typically, samples are collected by the preconcentrator for a fixed period of time (e.g., 2 minutes) before they are released for analyte separation and identification. Collecting for a fixed time period is a fundamental shortcoming of the chemical analysis process. When concentrations of potential toxins are high, precious time is wasted collecting excess sample material. Furthermore, this excess material will often saturate the preconcentrator and overwhelm a detector, necessitating cleaning before further analysis can resume. Conversely, when target analyte concentrations in the sample stream are low, insufficient analyte may be collected for detection or proper identification.
To avoid these problems, the present invention is directed to a mass-sensitive chemical preconcentrator that actively measures the mass of the sample on a microbalance during the collection process. The entire microbalance can then be rapidly heated to release the sample for further analysis. Therefore, the mass-sensitive chemical preconcentrator can optimize the sample collection time prior to release to enable the rapid and accurate analysis of analytes by the microanalytical system.